This invention relates to a spark plug cap adapted to be attached to a spark plug of an engine.
A motorcycle and a boat are subjected to large vibrations, and are exposed to fresh water or seawater, and therefore a spark plug cap having a core member mounted within a rubber body is used in these vehicles (see, for example, Japanese Patent Unexamined Publication Nos. 2-112187 and 3-254088). One example of such a spark plug cap is shown in FIG. 3.
In FIG. 3, a plug cap 102 having a perpendicularly-bent, tubular rubber body 120 is attached to an insulating portion 111 of a spark plug 101. A metal connection member 121 for contact with a plug terminal 112 of the spark plug 101 is mounted within the plug cap 102. The metal connection member 121 is connected to a high-voltage cord 103 (indicated in phantom) via a bar-like resistor 122 and a screw 123, thereby electrically connecting the spark plug 101 to the high-voltage cord 103.
The rubber body 120 includes a cap body 124, and a terminal body 125 formed integrally with the cap body 124. The cap body 124 is attached to the insulating portion 111 of the spark plug 101 to seal the outer periphery of the insulating portion 111. An end portion of the high-voltage cord 103 is attached to a distal end portion 125a of the terminal body 125, so that the outer periphery of the end portion of the high-voltage cord 103 is sealed by the distal end portion 125a. The outer peripheries of the insulating portion 111 and the high-voltage cord 103 are thus sealed, thereby preventing fresh water or seawater from intruding along the outer peripheral surfaces of the insulating portion 111 and the high-voltage cord 103.
A core member 126 made of a plastic material is mounted within the rubber body 120. The core member 126 surrounds the metal connection member 121 and the bar-like resistor 122. The core member 126 extends downwardly beyond the plug terminal 112 into the peripheral wall of the cap body 124, thereby increasing the rigidity of the cap body 124. With this construction, the plug cap 102 can be firmly connected to the spark plug 101, so that the shaking of the plug cap 102 due to vibrations can be prevented, and therefore the plug terminal 112, the metal connection member 121 and the high-voltage cord 103 are enhanced in durability.
As described above, the insulating portion 111 is held in sealing contact with the rubber body 120, and when the rigidity of the cap body 124 is increased as described above, the compressive force of the rubber disposed inwardly of the core member 126 increases to enhance the above sealing effect. Therefore, when the plug cap 102 is attached to the spark plug 101, the air within the plug cap 102 cannot easily escape, so that the air is compressed and is sealed in a space S between the spark plug 101 and the rubber body 120. Therefore, at the time of attachment of the plug cap 102 or during the operation of the engine, the plug cap 102 may be lifted or displaced, which results in a possibility that the connection metal member 121 fails to be held in proper contact with the plug terminal 112.